1. Field of the Invention
The invention relates generally to the field of instruments used in logging while drilling (“LWD”) oil wells in earth formations. More specifically, the invention relates to methods for determining the standoff of a well logging instrument from the wall of a wellbore using measurements made by a gamma-gamma density logging instrument.
2. Description of Related Art
Related art includes methods for determining the standoff of a well logging instrument from the wall of a wellbore by measuring gamma ray scattering, determining apparent formation density values, and compensating for materials interposed between the source and detectors other than the earth formation.
Wellbores are drilled through earth formations for extracting oil, gas, and water, and for other purposes. Wellbores are typically drilled using a rotary drill bit turned by a drilling rig, hydraulically operated motor (“mud motor”) or similar devices known in the art. After a wellbore is drilled through earth formation for oil, gas, or water extraction, a protective pipe or casing is typically cemented into the wellbore to maintain the mechanical integrity of the wellbore and to hydraulically isolate the penetrated earth formations from each other. When cementing the casing in place, it is useful to have knowledge of the shape of the wellbore, particularly its diameter along its length, so that the volume of cement needed to fill an annular space between the wellbore wall and the casing can be accurately determined. Various types of caliper devices are known in the art for determining the diameter of the wellbore, such as contact arm devices, and acoustic calipers. A typical contact arm device which can measure the diameter of the wellbore along its length is described in U.S. Pat. No. 3,321,625 issued to Wahl.
It has become common to measure petrophysical properties of the earth formations penetrated by wellbores, called “logging” the wellbore, while the drilling of the wellbore is in progress. See, for example, U.S. Pat. No. 5,513,528 issued to Holenka et al. which describes a method and apparatus for making petrophysical measurements during the drilling process. Such “logging while drilling” (LWD) instruments and methods include those which can make measurements corresponding to the bulk density of the earth formations penetrated by the wellbore. One such instrument is described, for example, in U.S. Pat. No. 5,473,158 issued to Holenka et al. One practical limitation of LWD instruments and methods is that using contact arm-type caliper devices to measure wellbore diameter, such as the one disclosed in the Wahl '625 patent, is extremely difficult and expensive. Consequently, acoustic travel time measurement devices, such as disclosed in the Holenka et al. '528 patent came into use.
More recently, U.S. Pat. No. 6,552,334 issued to Ellis et al. disclosed a method for determining the standoff of a well logging instrument from the wall of a wellbore by measuring gamma ray scattering, determining “apparent formation density” (as measured by X ray absorbance, and as contrasted to physical formation density), and compensating for materials interposed between the source and detectors other than the earth formation. The method of the '334 patent measures standoff based on a response related to “apparent formation density” derived from the counting rate of the longer spaced one of two detectors. The actual response in measuring “apparent formation density” includes an unwanted response component related to density of drilling mud between the source and detectors. The method of the '334 patent compensates for this unwanted response component by using a known value of the “apparent density of drilling mud” (as measured by X ray absorbance) and the difference in counting rate between the longer spaced one and the shorter spaced one of the two detectors.
Two disadvantages of the method of the '334 patent are found to be caused by sensitivity of the method to photoelectric-absorbing material in weighted mud. When mud is weighted with photoelectric-absorbing material like barite (barium sulfate, BaSO4), the “apparent density of the weighted mud” (as measured by X ray absorbance), exceeds its physical density. When the “apparent density of the weighted mud” exceeds its physical density, two disadvantages become evident. A first disadvantage is that if the “apparent density of the weighted mud” (which requires knowledge of the barite weight fraction in the mud) is unknown, the density caliper can be in serious error. A second disadvantage is that if the “apparent density of the weighted mud” approaches “apparent formation density”, the density caliper is unusable.